Fiber amplifiers can be optical fibers doped with rare earth elements, such as erbium. Such optical fibers, which are used in Erbium Doped Fiber Amplifiers (EDFA), have a central core formed of a silica matrix having erbium doping elements, possibly combined with additional doping elements to facilitate improved amplification.
Optical amplification in an EDFA-type optical fiber works by injecting into the optical fiber a pump signal that excites the doping element's ions (Er3+). When a light signal passes through this portion of optical fiber, it de-excites the ions by laser effect, producing a photon that is completely identical to the incident photon. In this way, the light signal is thus doubled.
Fiber amplifiers can also employ the Raman effect. Raman amplification does not use the atomic transitions of doped rare earth ions in the optical fibers but rather is based on an exchange of energy by Raman scattering.
Raman scattering is a process of inelastic scattering of the incident light in a material, which involves interactions with atomic vibrations of the matrix. Every material has a spontaneous Raman emission spectrum at given wavelengths (i.e., it behaves like a network of oscillators vibrating at a given frequency). The Raman spectrum of a glass, like that of a disordered amorphous material, is characterized by a large number of wavelengths forming a continuous spectrum over a wide frequency band. The intensity of the Raman emission increases with the input power applied to the material and becomes significant at a given power. This phenomenon is known as stimulated Raman scattering (SRS). It is thus possible, using the Raman emission of a material, to greatly amplify an optical signal by passing it through the material stimulated beforehand by an optical pump signal. In this kind of amplification, a pump photon is converted to another lower energy photon at the same wavelength as the signal.
A pump signal with a lower frequency than the signal to be amplified is used to ensure the amplification of an optical signal by SRS, with the frequency difference between the pump signal and the transmitted signal being approximately equal to the vibration frequency of the medium (typically 13.2 THz for silica). Thus, for stimulated Raman amplification of a 1550-nanometer signal, a 1450-nanometer pump signal is injected into the silica fiber.
The Raman amplification gain G, expressed in dB, can be expressed as follows:G=(10÷ln 10)·CR·Pp·Leff 
where                CR (in W−1·km−1) is the optical fiber Raman coefficient:CR=gR÷Aeff         wherein gR is the intrinsic Raman gain of the material (in km/W), and Aeff is the effective area of the optical fiber at the pump wavelength (in km2);        Pp is the power of the pump signal (in watts); and        Leff is the effective length of the optical fiber at the pump wavelength (in km), and can be specified by the equation:Leff=(1−e−αpL)÷αp         
wherein αp is the linear attenuation coefficient (in dB/km) and L is the length of the optical fiber.
Thus, to increase the efficiency of the Raman amplification (i.e., the Raman amplification gain G), it is possible to increase the intrinsic Raman gain of the material (gR) or to reduce the effective area of the optical fiber (Aeff), either of which gives rise to an increased Raman coefficient (CR). Alternatively, it is possible to reduce the optical losses (αp) at the pump wavelength or to increase the power of the pump (Pp).
Increasing the power of the pump (Pp) involves the use of costly lasers. This solution is impractical when low-cost, all-optical systems are sought.
An increase in the intrinsic Raman gain (gR) might be achieved by modifying the composition of the optical-fiber core, such as by adding germanium doping to a silica core or by producing optical fibers with a tellurite core rather than a silica core. This solution, however, leads to an increase in the optical transmission losses and poses a problem of compatibility with other optical fibers in an optical system that uses standard optical fibers.
Similarly, reducing the optical fiber's effective area (Aeff) to improve the Raman amplification gain (G) leads to problems of compatibility with existing optical transmission system standards and causes an increase in the optical transmission losses.
It has also been proposed to improve the Raman amplification gain (G) by adding rare earth dopants to the optical-fiber core. This solution, however, does not produce satisfactory results because of the absorption of the signal by the rare earth ions.
Finally, the production of a Raman fiber amplifier requires a compromise between the Raman coefficient (CR) on the one hand and the optical losses of the optical fiber on the other. In the literature, this compromise is often estimated by the Figure of Merit (FOM), expressed in W−1·dB−1, which represents the ratio between the Raman coefficient of the fiber (CR) expressed in W−1·km−1 and the optical losses in the optical fiber (αp) expressed in dB·km−1 at the wavelength of the pump. Typically, for standard single-mode fibers (e.g., having with a silica core containing less than 5 weight percent germanium and an effective area of 80 μm2) intended for Raman amplification, the Raman FOM is limited to 3.2 W−1·dB−1.
Consequently, there is a need for an optical fiber amplifier that has an improved Raman FOM and yet is compatible with standard optical fibers.
An electromagnetic wave, such as light propagating in the optical fiber, can polarize the electron cloud surrounding nanostructures that are present in the optical-fiber core, thus creating coherent collective oscillation (known as “surface plasmon”). When a wavelength is injected under resonance conditions with the oscillation wavelength of this polarized cloud, the energy can be transferred to this wavelength. The resonance wavelength of the nanostructures depends on their shape and size and the nature of the metal of the nanostructure.
The phenomenon of Surface Plasmon Resonance (SPR) has been observed.
For example, the publication “Optical Properties of Gold Nanorings” by J. Aizpurua et al., Physical Review Letters, Vol. 90, No. 5, Feb. 7, 2003, describes the optical response of ring-shaped gold nanoparticles arranged in a glass matrix.
The publications “Nanoengineering of Optical Resonances” by S. J. Oldenburg et al., Chemical Physics Letters, May 22, 1998, pp. 243-247, and “A Hybridation Model for the Plasmon Response of Complex Nanostructures” by E. Prodan et al., Science, Vol. 302, Oct. 17, 2003, describe different shapes and compositions of nanoparticles and the resulting optical resonance.
The publication “Symmetry Breaking in Individual Plasmonic Nanoparticles” by Hui Wang et al., PNAS, Vol. 103, No. 29, Jul. 18, 2006, describes nanoparticles composed of a dielectric core and a metallic shell. This publication describes more specifically the effect of the size of the metallic shell of the nanoparticles on the surface plasmon resonance shift.
The publication “Surface-Enhanced Raman Spectroscopy” by R.L. Garell, in Analytical Chemistry, 1989, 61, pp. 401-411, describes a molecule characterization technique using SRS amplification with silver nanoparticles in solution.
Furthermore, the publications “Surface Plasmon Polariton Modified Emission of Erbium in a Metallodielectric Grating” by J. Kalkman et al., Applied Physics Letters, Vol. 83, No. 1, Jul. 7, 2003, “Coupling of Er ions to Surface Plasmons on Ag” by J. Kalkman et al., Applied Physics Letters, Vol. 86, 2005, 041113-1-3, and “Plasmon-Enhanced Erbium Luminescence” by H. Mertens et al., Applied Physics Letters, Vol. 89, 2006, 211107-1-3, describe an increase in the light intensity emitted by the erbium ions arranged in proximity to silver nanoparticles. It is thus possible to reduce the thermal effects in a planar guide.
Metallic nanoparticles have also been used for optical sensors. For example, U.S. Pat. Nos. 6,608,716 and 7,123,359, each of which is hereby incorporated by reference, describe optical sensors having microcavities constituted of a dielectric substance doped with metal, semi-metal, and/or semi-conductor atoms and having a plurality of nanoparticles aggregated to form a fractal structure. U.S. Pat. No. 6,807,323, which is hereby incorporated by reference, describes an optical sensor using the phenomenon of Surface Plasmon Resonance (SPR) between a thin conducting film and a thin dielectric film doped with rare earth elements or transition metals.
The phenomenon of Surface Plasmon Resonance (SPR), however, has not been used to improve the Raman gain of a fiber amplifier. The manufacturing constraints of optical fibers impose choices on the nature, size, and shape of the incorporated nanostructures.
Optical fibers including nanoparticles are generally known. For example, European Patent No. 1,347,545 (and its counterpart U.S. Pat. No. 7,031,590), or International Publication No. 2007/020362 (and its counterpart U.S. Patent Publication No. 2009/0116798), all of which are hereby incorporated by reference, describe optical fibers including nanoparticles in the optical-fiber core. The nanoparticles described in these publications include a rare earth doping element and at least one element improving the amplification of the signal, such as aluminum, lanthanum, antimony, bismuth, or other element.
These foregoing documents, however, do not describe metallic nanoparticles allowing for the creation of a phenomenon of Surface Plasmon Resonance (SPR) in the core of an optical fiber.
As noted, a need exists for an optical fiber amplifier that has an improved Raman FOM while retaining compatibility with standard optical fibers.